This invention relates to filtration. More particularly, this invention relates to coalescence filtration. Specifically, this invention relates to removal of liquid from coalescence filters using a temperature gradient to cause the migration of liquid droplets from the fibers of the filter.
There are a number of applications where it is desirable or even necessary to remove a liquid component of a gas stream. These include industrial applications such as electric power generation, coal gasification, distillation, refrigeration and the like. In such applications, presence of liquid in a gas stream may interfere with the operation of downstream equipment or processes. Alternatively, it may be desired to remove a liquid component from a gas stream prior to release into the atmosphere, or because the recovered liquid is valuable.
Previous methods for removal of liquid from a gas stream have included traditional coalescence filters, inertial separators and sonic agglomerators. In coalescence filters, droplets of liquid are collected by a filter and must grow to a relatively large size before they are removed by gravity or by aerodynamic forces associated with the gas flow. The pressures necessary for aerodynamic forces to remove accumulated droplets may be so large that the mechanical structure of the filter is degraded. Inertial separators, such as cyclone separators, are generally designed to separate liquid particles from a gas stream by inducing a change in the path of the gas stream and driving the liquid particles against a surface. Liquid droplets may be separated from the carrier gas stream by centrifugal forces and collected separately from the gas. Sonic agglomerators utilize sound waves to cause relatively small particles to agglomerate, forming larger particles which may be more easily removed by other methods such as use of a cyclone separator. Electrostatic precipitators may also be used to separate liquid particles from a gas stream. The above methods are of limited use in certain applications however, such as those involving high temperatures or pressures.
Another method for the filtration of a gas stream which contains liquid particles includes heating the filter to cause the liquid particles to be vaporized, thereby preventing the filter from becoming fouled by accumulated liquid. This method cannot be utilized for inflammable organic solvents or other compounds which would be chemically unstable at the elevated temperature present in the filter. Therefore, there is a need for an alternative method for the filtration of a gas stream which contains liquid particles or vapor.
There are several known models which attempt to explain the behavior of droplets of liquid on a fiber when subjected to a temperature gradient. One of these models uses Marangoni convection. According to this model, surface tension of a liquid typically decreases when temperature increases. Accordingly, Marangoni convection will cause droplets of a liquid on a fiber to move toward the hot end of the fiber.
It can also be speculated that if a droplet on a fiber is subjected to a temperature gradient, the contact angle of the liquid on the hot side of the droplet would increase and become larger than the that on the cold side. This could cause an asymmetry in the drop shape to arise, and a force displacing the droplet toward the cold side would emerge. This model has been shown to be inaccurate, however, because the contact angle of a liquid has been shown to typically decrease with temperature.
Vapor recoil may also be used to attempt to explain movement of a droplet in a temperature gradient. According to this model, evaporating liquid on the hot end of a droplet would create a force which would push a droplet toward the cold end of the fiber. This model, however, does not explain the movement of a droplet in the absence of significant evaporation.